Originally Posted by niznai
Remember that while most of the above methods are correct there are variables such as tire diameter (which in real life is not identical between tires to the precision some people claim they achieve when measuring droop) there is suspension slop, air gap and so on.
I think the most accurate way of measuring is to lift the car up (as if using a floor jack) and measuring how far you lift before tires leave the ground. This is of course easier said than done so usually I try to compromise somehow until the car looks right.
The easiest is to measure axle height (to eliminate the extra variable introduced by measuring under some part of the suspension arm which relies too much for my liking on arms being moulded perfectly identical) with the chassis on blocks but as I said, once the tires are on, there can be differences, so I check quickly if they lift at the same time off the ground and if they don't, I readjust.
Using setup wheels again introduces another horde of variables because you don't know if the wheels are exactly the same diameter as your tires, plus your tires compress (so ground clearance will be different) and then when unloaded, they spring back, another unknown quantity (depending on insert, air gap and manufacturing tolerance). You've really got to find where enough is enough for you and try to keep consistent.
If you know what ride height with your setup wheels = what ride height with each tyre, then setup wheels give you very precise settings. I have found using setup wheels to measure droop (which is the base for setting tweak) gives me a chassis that tracks very straight on and off power compared to any other way I have measured droop.